Terminals for communication are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. The terminals, herein also denoted wireless nodes, are enabled to communicate wirelessly in a cellular communications network, sometimes also referred to as a cellular radio system or cellular network, or other wireless communications network or system. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNodeB (eNB), NodeB, B node, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. The base stations and terminals involved in communication may also be referred to as transmitter-receiver pairs, where the respective transmitter and receiver in a pair may refer to a base station or a terminal, depending on the direction of the communication. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission in LTE is controlled by the radio base station.
In a hierarchical telecommunications network the backhaul portion of the network comprises the intermediate links between the core network, or backbone network, and the small sub networks at the “edge” of the entire hierarchical network.
In wireless communication systems, co-channel interference is one of the main factors that limits the capacity of the wireless communication system. There exists several approaches for dealing with the interference in multi-user scenarios, which require different levels of coordination among the transmitters. Interference Alignment (IA) is one such technique which can mitigate the interference between transmitter-receiver pairs. Another approach is interference suppression in a receiver.
The basic idea behind IA is that the multiple transmitters seek to align the caused interference to unintended receivers within a minimum dimension subspace, so that at each receiver the remaining dimensions can be used for interference-free communication. The minimum dimension of the interference subspace depends among other things on the number of transmit antennas. The more transmit antennas, the more options are there to align the interference. This allows each receiver to eliminate all the interference by simply canceling everything that falls into this subspace. This is a rather general idea, in the sense that the signals can be aligned in any given dimension, such as time, frequency, or space. There are also several possible ways to specify and implement IA algorithms, depending on the cost function to be optimized and on the degree of coordination.
In a conventional cellular system, as an example, only the intra-cell interference is mitigated. When there exists coordination between different cells such as in a Coordinated Multi-Point (CoMP) system, the interference between cells, i.e. inter-cell interference, can be canceled by using techniques such as joint transmission. Joint transmission requires tight coordination between the transmitters and that the transmitted streams are shared among the transmitters. However, when it is not possible to perform a joint transmission, such as in the case of loose coordination, then IA would be a suitable candidate for canceling the inter-cell interference.
Other interference avoiding techniques are evolved such as Minimum Mean Squared Error (MMSE) based IA which reduces the interference subspace rather than completely aligning all interference. The interference subspace is defined herein as the signal subspace of the interfering signals.
Interference suppression in a receiver is dependent on the number of receiver antennas and receiver implementation. Different wireless nodes, e.g. UEs, may have different interference suppression capability and performance dependent on receiver and antenna design.
In practice it is difficult to coordinate all transmissions to achieve interference alignment. To give substantial gain, large clusters of cells need to be coordinated and the transmitters need to be equipped with a large number of antennas. The co-ordination also needs to be done with short delay requiring very good backhaul.
Further, the wireless node receiver implementation is typically not specified by standard (3GPP, IEEE nor others) but left open for vendors to enable continuous improvement. Receivers including interference suppression can be implemented in different ways, for example by Maximum Ratio Combining (MRC), MMSE or zero forcing. For a given received signal direction and phase, the different solutions will be capable of suppressing interference unequally good and will also differ in robustness for interference from different directions and with different phases. This results in that best phase for interference reception differ between different UE vendors and even between different versions of UEs from the same UE vendor. The impact from receiver suppression and UE or wireless node implementation is therefore difficult to take into account when applying interference alignment, as receiver implementation, capability to suppress interference and best known reception phase in the wireless node may be unknown to a radio node such as a base station scheduling transmissions that may generate interference to the wireless node. There is thus a need for solutions that facilitate interference alignment in a wireless communications network.